Method for decontaminating solid iodine filters

ABSTRACT

The invention relates to a process for decontaminating solid iodine filters containing silver iodide, silver iodate and/or physisorbed molecular iodine. 
     This process consists in placing the filter in contact with an aqueous solution of a reducing agent chosen from hydroxylamine, hydroxylamine salts, ascorbic acid, ascorbic acid salts, ascorbyl esters, sodium borohydride, sodium hypophosphite, formaldehyde, urea, formic acid and mixtures thereof, to extract the iodine from the filter and dissolve it in the aqueous solution. 
     The dissolution of the silver, in the solution of reducing agent or in another suitable aqueous solution, may also be ensured, simultaneously or successively.

TECHNICAL FIELD

The present invention relates to a process for decontaminating solidiodine filters used in the nuclear industry.

In plants for reprocessing spent nuclear fuels, the recovering of theresidual iodine contained in the gaseous effluents in the form ofmolecular iodine I₂ and/or of organoiodine compounds such as iodoalkanesor alkyl iodides, for example CH₃I, is performed, after discharging thegaseous effluents, by means of solid mineral traps commonly known asiodine filters. These are circular cartridges filled with porous aluminaor silica beads, impregnated with silver nitrate, these beadsconstituting the actual filter. In these iodine filters, the iodinereacts with the silver nitrate to form iodine compounds such as silveriodide and silver iodate, possibly with a small presence of physisorbedmolecular iodine I₂, silver iodide being predominant and sparinglysoluble in water.

These iodine filters constitute a solid waste contaminated with iodine¹²⁹I, which cannot be directly surface-stored, and for which no matrixis currently available for deep-level storage.

It would therefore be advantageous to decontaminate these filters inorder to remove the iodine they contain, which would allow thedeclassification of the waste for admission into surface storage afterconditioning in a suitable matrix. For conditioning of this type, forexample in a cement matrix, the residual admissible iodine content is1.3 or 6.3 mg of iodine per g of filter, depending on the packagechosen.

PRIOR ART

G. Modolo and R. Odoj described in Proc. International Conference onEvaluation of Emerging Nuclear Fuel Cycle Systems (Global 1995), 11 to14 Sep. 1995, Versailles, France, Vol. 2, pp. 1244–1251 [1] and inNuclear Technology, Vol. 117, 1997, p. 80–86 [2], the separation ofiodine using iodine filters of this type by extraction with sodiumsulphide or hydrazine, or by reduction with hydrogen.

The object of the studies reported by Modolo et al. was solely therecovery in solution of the maximum amount of iodine 129 in order totransmute it in a reactor and thus reduce its duration of harmfulness,rather than the decontamination of the porous solid support.

In the case of sodium sulphide, the silver iodide is converted intoinsoluble silver sulphide and into soluble sodium iodide, but thisprocess has the drawback of producing effluents containing sulphidesthat can cause corrosion of the plants and unwanted precipitations.

In the case of a reductive heat treatment with hydrogen, obtaining anadvantageous decontamination factor (170 i.e. 0.77 mg of iodine/g offilter, i.e. 0.6% of the initial content of 128 mg of iodine per g offilter) requires the process to be performed at a temperature above 500°C. for 6 hours, but then comes up against the increasing volatilizationof AgI. The volume content of hydrogen in the gas mixtures used (N₂—H₂at constant rate) is from 10% to 100%, which poses certain problems onthe industrial scale, given the risks associated with using hydrogen andwith corrosion.

In the case of hydrazine, aqueous hydrazine solutions are used asreducing agent to reduce the Ag⁺ cation to silver metal, releasingiodine in the form of soluble iodide. However, the lowest residualiodine content observed is 1.9 mg of iodine per g of filter, i.e. 1.5%of the initial content of 128 mg of iodine per g of filter, and adecontamination factor of 67.

Furthermore, this process is laborious since it involves repeatedtreatments (several successive washes with highly concentrated solutionsof hydrazine-based, N₂H₄ at 5 mol.L⁻¹, interspersed with sequences ofdecantation and then filtration of the solid beads).

Moreover, the authors observe purple vapours above the solution,corresponding to a release of iodine that is therefore not entirelyrecovered in solution in the form of iodide, thus necessitating atreatment of the gases to recover the iodine. Thus, this process doesnot producte an iodine separation that is sufficient to declassify thiswaste. Furthermore, the hydrazine may lead to the formation of unstableazides, with risks of explosion.

The subject of the present invention is, specifically, ahydrometallurgical (wet-route) process for treating an iodine filter,which produces a sufficient decontamination of solid iodine filters, forexample a residual iodine content of less than or equal to 1.1 mg ofiodine per g of filter for an iodine-saturated filter, which has aninitial content of 140 mg of iodine per g of sorbent.

DESCRIPTION OF THE INVENTION

One subject of the invention is a process for decontaminating a solidiodine filter containing silver iodide, silver iodate and/or physisorbedmolecular iodine, which consists in placing the filter in contact withan aqueous solution of a reducing agent chosen from hydroxylamine,hydroxylamine salts, for example hydroxylammonium nitrate, ascorbicacid, ascorbic acid salts such as sodium ascorbate, ascorbyl esters suchas ascorbyl palmitate, sodium borohydride, sodium hypophosphite,formaldehyde, urea, formic acid and mixtures thereof, to extract theiodine from the filter and dissolve it in the aqueous solution.

According to the invention, a reducing agent is thus used to convert theiodine compounds included in or present on the solid filter into asoluble portion consisting of iodide anions and into an insolubleportion consisting of silver in metallic form, which remainspredominantly in the pores of the filter.

In the process of the invention, the choice of a reducing agent otherthan the hydrazine used in documents [1] and [2] makes it possible toavoid the safety problems posed by using this reagent, i.e. itsinstability in nitric solution, associated with the potential formationof explosive azides. Furthermore, the destruction of the spent hydrazinesolutions is difficult and generates unwanted effluents, in particularduring the use of nitrites in large excess in acidic medium.

On the other hand, the use, in accordance with the invention, ofhydroxylamine and especially of ascorbic acid (or vitamin C) and also ofthe salts and derivatives thereof, does not present these drawbacks.Ascorbic acid is entirely safe to use. As regards hydroxylamine, onlyvery specific concentration, temperature and confinement conditions canresult in violent reactions, but these may be readily avoided by meansof the elementary precautionary rules adhered to by any good operator(in particular satisfactory venting); there is no generation of azidesin any case. These reagents are easy to destroy. It is not obligatoryfor ascorbic acid, but may be performed in basic medium at moderatetemperature; hydroxylamine, in dilute solution, may also be degraded inalkaline medium, like ascorbic acid, or by mild oxidation, for example,with aqueous hydrogen peroxide solution in slightly acidified medium.Ascorbic acid and hydroxylamine are therefore entirely compatible withthe processing of the effluents from factories for reprocessing spentnuclear fuels.

Furthermore, the process of the invention is easy to carry out and leadsto a residual iodine content that is markedly lower than the normsadmissible to allow the declassification of waste.

The solid filters that may be processed by the process of the inventionmay be of various types, and may comprise a mineral or organic support.Examples of mineral supports that may be mentioned include ceramics, inparticular porous ceramics, such as silica, alumina, other ceramicoxides, and also carbides and nitrides.

Examples of organic supports that may be mentioned include polymersupports consisting of organic resins, for example ion-exchange resins.

These supports are impregnated with silver nitrate, which reacts withiodine to form iodine compounds such as silver iodide and silver iodate.

Thus, the iodine filter becomes charged with radioactive iodineoriginating from gaseous effluents such as those from plants for thereprocessing of spent nuclear fuels. It generally consists of poroussilica or alumina beads, containing silver iodide, silver iodate and/orphysisorbed molecular iodine.

According to the invention, the following reduction reactions arecarried out to recover the iodine in aqueous solution:

Agl + e′ → Ag_((c)) + l⁻ _((aq)) (half-reaction 1) standard potential,E⁰ ₁ = −0.1522 V/ENH AgIO₃ + e⁻ → Ag_((c)) + IO₃ ⁻ _((aq))(half-reaction 2) standard potential, E⁰ ₂ = +0.354 V/ENH Ag⁺(aq) + e⁻ →Ag_((c)) (half-reaction 3) standard potential, E⁰ ₃ = +0.7991 V/ENHIO₃′ + 3H₂O + 6e⁻ → I⁻ + 6 OH⁻ (half-reaction 4) standard potential, E⁰₄ = +0.257 V/ENH (in basic medium)

-   -   in basic medium, molecular iodine dismutes to iodide and iodate,        and the reduction thus comes down to that of the IO₃ ⁻ anion.

To carry out these reactions, any reducing agent with a standardpotential or an apparent potential, in a suitable pH range, of less thanE⁰ ₁ (which is the lowest potential) is suitable if the reductionmechanism directly involves silver iodide AgI. If it is in fact the Ag⁺cation resulting from the dissociation of AgI that is reduced, thepotential of the reducing agent must then be less than E⁰ ₄ in order forit to be oxidized by all the iodine and silver species present, whichcorrespond to the half-reactions 2, 3 and 4. If the reduction of theiodate directly involves AgIO₃, the maximum potential of the reducingagent may theoretically be raised to E⁰ ₂.

The reducing agents used in the invention satisfy these characteristicsand are thus suitable for dissolving iodine in the form of iodide in theaqueous solution. Among these reducing agents, hydroxylamine and sodiumascorbate are preferred.

Hydroxylamine has the advantage of giving inert gaseous oxidationproducts, namely nitrogen (N₂) and nitrous oxide (N₂O).

It may also be noted that the use of reducing agents such ashydroxylamine and ascorbic acid, and the salts and derivatives thereof,is advantageous since they belong to the family of water-soluble organiccompounds containing only C, H, O and N, which may be destroyed withoutformation of corrosive products and without increasing the saline chargeof the effluent solutions.

To perform the process of the invention, it suffices to immerse thesolid iodine filter in the aqueous solution of reducing agent having asuitable pH. The aqueous solution may also be circulated through thefilter.

The aqueous solution generally used is one whose pH is adjusted to avalue in the range from 10 to 14, or even more basic with an OH⁻ ionconcentration that may be up to 2 or even 3 mol.L⁻¹.

This may be performed using a mineral base, for example sodiumhydroxide, or a water-soluble organic base such as tetramethylammoniumhydroxide, aqueous ammonia or the like. This solution is chosen asreducing agent to limit, preferably, the degradation of the support, thepartial dissolution of which during the processing might lead to anuntimely reprecipitation in the following stages of processing of thesolutions.

Preferably, a sodium hydroxide solution with a pH ranging from 10 to 14,or even more basic with an OH⁻ ion concentration that may be up to 2 oreven 3 mol.L⁻¹, is used.

The reducing agent concentration of this aqueous solution is chosen soas to ensure solubilization of the iodine under the best conditions.This concentration is generally from 0.5 to 2 mol.L⁻¹.

For the placing in contact, from 40 to 250 ml of aqueous solution per 10g of filter, i.e. from 40 to 250 g of filter per litre of solution ispreferably used: 250 g/L corresponding approximately to the minimumvolume to submerge the filter.

The reductive dissolution treatment may be performed at room temperatureor at a higher temperature, preferably at a temperature from 20 to 60°C., for a period of from about 15 minutes to 4 hours, for example fromabout 30 minutes to 2 hours.

By working under these conditions, the residual iodine content of thesolid filter is less than or equal to 1.1 mg per g of filter, whichcorresponds to a decontamination factor of greater than or equal to 127for an initial content of 140 mg of iodine per gram of filter. Thesilver content is between 100 and 120 mg per g of filter for an initialcontent of about 125 mg.

After this placing in contact of the solid iodine filter with theaqueous solution of reducing agent, the decontaminated filter isgenerally rinsed with water or an aqueous solution having a pH ofgreater than or equal to 7.

This rinsing may be followed, if necessary, by drying, for example bysimple draining or by circulation of air, depending on the residualmoisture level that is admissible for the conditioning for the purposeof surface storage.

When the solid iodine filter contains unconverted silver nitrate, apretreatment to dissolve the silver nitrate in a dilute acid solutionsuch as 0.1M nitric acid solution may be performed before performing thereductive treatment. This makes it possible to dissolve the unconvertedsilver nitrate and thus to reduce the amount of reagents subsequentlyemployed, which would otherwise also serve for the reduction of thesilver nitrate to silver metal, needlessly consuming a fraction of thereductive silver.

If it is performed, this pretreatment must be followed by carefulrinsing of the iodine filter with water to reduce the subsequentconsumption of base.

According to one working variant of the process of the invention, if itis desired to perform a more thorough decontamination of the filter,dissolution of the silver present in the filter into an aqueous solutionis also performed.

This may be performed, after having separated the solid filter from theaqueous solution of reducing agent, by placing the filter thus separatedin contact with a silver-dissolving solution.

Solutions that may be suitable for use are, for example, oxidativeacidic solutions. A nitric solution or a solution with a redox potentialof greater than +0.7991 V/ENH, which would not oxidize the iodide ion,may be used in particular.

This solution may be a nitric acid solution, with a nitric acidconcentration of from 2 to 6 mol.L⁻¹.

This dissolution of the silver in the aqueous solution of reducing agentmay also be performed simultaneously, for example by adding to thisaqueous solution a silver-complexing agent, for example potassiumcyanide.

When the reductive treatment and the dissolution of the silver areperformed successively, the cycle comprising the steps below may beperformed at least twice:

-   -   a) reductive treatment of the iodine filter with an aqueous        solution of the reducing agent, having a pH of from 10 to 14, or        even more basic with an OH⁻ ion concentration that may be up to        2 or even 3 mol.L⁻¹,    -   b) separation of the filter from the aqueous solution, and    -   c) dissolution of the silver by immersing the filter separated        out in step b) in a nitric acid solution with a nitric acid        concentration of from 2 to 6 mol.L⁻¹.

In this case, the iodine filter is alternately immersed in a reductiveaqueous solution and in an acidic aqueous solution to perform one ormore reduction-dissolution cycles.

The reductive solution reduces the silver iodide to silver metal, whichremains predominantly in the pores of the solid support, partiallyblocking them and limiting the decontamination process. The effect ofthe nitric acid solution is to dissolve the silver metal, thus allowingthe reduction reaction to proceed to the next cycle.

Two or three cycles with or without final nitric washing may beperformed, resulting in a very substantial lowering of the residualiodine content in the filter.

Preferably, in this embodiment of the process of the invention, carefulrinsing of the solid iodine filter in water is performed after eachreductive or acidic treatment, to reduce the consumptions of base and ofnitric acid, since an acidic reductive solution is inoperative.

When the reductive treatment and the dissolution of the silver areperformed simultaneously, the iodine filter is treated in an aqueoussolution having a pH of from 10 to 14, or even more basic with an OH⁻ion concentration that may be up to 2 or even 3 mol.L⁻¹, containing thereducing agent and a cyanide such as potassium cyanide, tosimultaneously dissolve the silver metal in the aqueous solution.

This simultaneous dissolution is advantageous compared with thesuccessive reductive and dissolution treatments since it requires onlyone solution and thus makes it possible to avoid the alternation ofreductive treatment and of acidic washing. This is reflected by fewerrinses and a marked reduction in the volumes of solutions used and inthe volumes of effluents to be subsequently treated.

The process of the invention is advantageous since it is efficientenough not to require the opening of the cartridge containing themineral trap beads. At the very most, a forced circulation of thesolutions through the cartridge can facilitate and accelerate thedecontamination operation; otherwise, simple dipping may suffice. Afterrinsing and drying, the decontaminated filter may be conditioneddirectly in cement. The formation of mineral fines from the supportbeads (partial disintegration) may necessitate a filtration of thesolutions used.

For the sake of safety, during the acidic washing (prewashing of thefilter to dissolve the unconverted silver nitrate and dissolution of thesilver metal between two reductive treatments), it is possible to guardagainst a potential release of iodine by performing a basic washing ofthe gases, the effluent of which may be mixed with the solution used forthe reductive treatment.

Other characteristics and advantages of the invention will emerge moreclearly on reading the examples that follow, which are obviously givenas non-limiting illustrations.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Examples 1 to 5 given below relate to the processing, via the process ofthe invention, of 10 grams of filter consisting of porous alumina beadscontaining 140 mg of iodine per g of charged filter. The filter containsabout 12.5% of elemental silver by mass, initially in the form ofnitrate; 140 mg of elemental iodine per gram of filter correspond tosaturation of the filter, i.e. to the total conversion of the silvernitrate to solid silver iodide and silver iodate in the porosity. Thereducing agents used are sodium ascorbate (Examples 1 to 4) andhydroxylammonium nitrate or HAN (NH₃OH⁺, NO₃ ⁻) in Example 5.

EXAMPLE 1

The reductive treatment is performed by immersing the filter (containedin a metal gauze envelope similar to that of the actual filtercartridge) in 40 ml of reductive solution containing 2 mol of ascorbateper litre, pH=14, for 4 hours at 60° C.; there is no circulation of thesolution or movement of the beads. The ratio of the mass of filter tothe volume of solution is 250 g per litre. The reductive solution isremoved and, with the filter still in its envelope, rinsing is performedwith water or with a solution of pH≧7. The residual iodine content inthe filter is then measured: it is 0.8 mg per gram, which corresponds toa decontamination factor of 175.

EXAMPLE 2

The reductive treatment is performed by immersing the filter (containedin a metal gauze envelope similar to that of the actual filtercartridge) in 250 ml of reductive solution containing 0.5 mol ofascorbate per litre, pH=14, for 4 hours at 60° C.; there is nocirculation of the solution or movement of the beads. The ratio of themass of filter to the volume of solution is 40 g per litre. Thereductive solution is removed and, with the filter still in itsenvelope, rinsing is performed with water or with a solution of pH≧7 andthen drying. The residual iodine content in the filter is then measured:it is 1.1 mg per gram, which corresponds to a decontamination factor of127.

EXAMPLE 3

The reductive treatment is performed by immersing the filter (containedin a metal gauze envelope similar to that of the actual filtercartridge) in 250 ml of reductive solution containing 2 mol of ascorbateper litre, pH=11, for 4 hours at 60° C.; there is no circulation of thesolution or movement of the beads. The ratio of the mass of filter tothe volume of solution is 40 g per litre. The reductive solution isremoved and, with the filter still in its envelope, rinsing is performedwith water or with a solution of pH≧7 and then drying. The residualiodine content in the filter is then measured: it is 0.9 mg per gram,which corresponds to a decontamination factor of 156.

EXAMPLE 4

The reductive treatment is performed by immersing the filter (containedin a metal gauze envelope similar to that of the actual filtercartridge) in 40 ml of reductive solution containing 2 mol of ascorbateper litre, pH=11, for 4 hours at 60° C.; there is no circulation of thesolution or movement of the beads. The ratio of the mass of filter tothe volume of solution is 250 g per litre. The reductive solution isremoved and, with the filter still in its envelope, rinsing is performedwith water or with a solution of pH≧7 and then drying. The residualiodine content in the filter is then measured: it is 0.7 mg per gram,which corresponds to a decontamination factor of 200.

EXAMPLE 5

The reductive treatment is performed by immersing the filter (containedin a metal gauze envelope similar to that of the actual filtercartridge) in 250 ml of reductive solution containing 2 mol ofhydroxylammonium nitrate or HAN (NH₃OH⁺, NO₃ ⁻) per litre, pH=13, for 4hours at 25° C.; there is no circulation of the solution or movement ofbeads. The ratio of the mass of filter to the volume of solution is 40 gper litre. The reductive solution is removed and, while the filter isstill in its envelope, rinsing is performed with water or with asolution of pH≧7.

The residual iodine content in the filter is then measured: it is 2.0 mgper gram, which corresponds to a decontamination factor of 70.

Efficient decontamination is thus observed in a single attack at roomtemperature, which is very close to that obtained by Modolo et al. withseveral attacks at elevated temperature.

The results obtained in Examples 1 to 5 show that the use of a singlereductive basic wash by simple dipping of the filter, using reducingagents consisting of ascorbic acid or ascorbic acid salts, ascorbylesters or hydroxylamine and its salts, affords a real advantage over theprocess used by Modolo starting with hydrazine.

Thus, the process of the invention already makes it possible to gobeyond the specifications imposed for surface storage, since thecontents measured during the tests reach values as low as 0.7 mgI/g offilter (decontamination factor=200 for an initial iodine content of 140mg.g⁻¹) or slightly higher depending on the operating conditions. Thesevalues are always about half the size of those obtained by Modolo et al.with the process using hydrazine, which, moreover, would not beefficient enough to allow the declassification of the filters in all thecases of conditioning.

Moreover, the process of the invention consists of a single reductivebasic wash by simple dipping of the whole filter cartridge in a tank,which is extremely simple to carry out (absence of alternate sequencesof decantation/filtration/washing as proposed by Modolo).

Finally, no purple vapours derived from the untimely or uncontrolleddesorption of iodine were ever observed in the gases during theimplementation of the process with the abovementioned reducing agents,given the pH used, which is high enough to bring about the dismutationand/or reduction of the molecular iodine.

Examples 6 to 9 below illustrate a thorough decontamination of iodinefilters by performing the working variant of the process of theinvention.

EXAMPLE 6

In this example, the working variant of the process of the inventionalso comprising the dissolution of the silver is used, to process a 70kg iodine filter, consisting of porous alumina beads, containing 140 mgof iodine per g of Al₂O₃ (filter containing about 12.5% silver by mass,initially in the form of nitrate; 140 mg of iodine per gram of Al₂O₃correspond to saturation of the filter, i.e. the total conversion of thesilver nitrate to silver iodide and silver iodate).

In the first step, the reductive treatment is performed by immersing theiodine filter in a sodium hydroxide solution with a pH of 13, containing2 mol/L of hydroxylamine, at a temperature of 60° C., for about 30minutes. The iodine filter is then removed from the solution, and rinsedwith water, and the second step of acidic treatment is then performed byimmersing the rinsed iodine filter in an aqueous solution containing 6mol/L of nitric acid, at a temperature of 60° C., for about 15 minutes.The iodine filter is then removed from this solution and is subjected torinsing with water.

The whole treatment cycle described above comprising the reductivetreatment and the acidic treatment is repeated twice.

At the end of the operation, the residual iodine content of the iodinefilter is less than 0.03 mg of iodine per gram of the solid support (30ppm) and its silver content is of the same order but slightly higher,i.e. less than or equal to 100 ppm.

The maximum volumes of solutions required for the treatments and rinsesare of the order of one m³ for this 70 kg filter.

The silver present in the nitric solutions derived from the dissolutiontreatment represents about 8.4 kg. It may thus be almost quantitativelyrecovered.

EXAMPLE 7

The same procedure as in Example 6 is followed, to process an identicalfilter, but a sodium hydroxide solution of pH 13 containing 2 mol/L ofsodium ascorbate instead of hydroxylamine is used for the reductivetreatment.

The results obtained are identical to those of Example 6.

EXAMPLE 8

In this example, a 70 kilogram iodine filter, containing 140 mg ofiodine per g of Al₂O₃ is processed by simultaneously performing thedissolution of the silver.

In this case, the filter is immersed in a sodium hydroxide solution ofpH 13 containing 2 mol/L of hydroxylamine and 4 mol/L of potassiumcyanide KCN, at a temperature of 60° C. for about four hours, afterwhich it is extracted from the solution and rinsed.

Under these conditions, a filter whose residual iodine content is lessthan or equal to 0.030 mg of iodine per gram of filter (i.e. 30 ppm) isobtained at the end of the operation. The silver is also almostquantitatively recovered in the sodium hydroxide solution.

EXAMPLE 9

The same procedure as in Example 8 is followed, to process an identicalfilter, but sodium ascorbate is used instead of hydroxylamine, at aconcentration of 2 mol/L.

Equivalent Results are Obtained.

The process of the invention is thus very advantageous since it makes itpossible to achieve a very high level of decontamination while at thesame time being easy to perform since the decontamination is carried outin aqueous solution in a simple tank.

Moreover, the effluents generated by this process are compatible withthe effluents from factories for reprocessing nuclear fuels.

In the variant of the process according to which the silver is dissolvedby means of cyanide, it is necessary to ensure that the solution willnever be subsequently acidified, so as to avoid the release of hydrogencyanide. The best approach is to destroy the cyanide immediately afterthe decontamination and separation of the decontaminated filter. Thismay be performed by adding excess ferrous sulphate, which gives stablehexacyanoferrate(II) complexes.

REFERENCES CITED

[1] G. Modolo and R. Odoj, Proc. International Conference on Evaluationof Emerging Nuclear Fuel Cycle Systems (Global 1995), 11 to 14 Sep.1995, Versailles, France, Vol. 2, pp. 1244–1251

[2] Nuclear Technology, Vol. 117, 1997, p. 80–86

1. A process for decontaminating a solid iodine filter comprising silveriodide, silver iodate and/or physisorbed molecular iodine, whichcomprises: placing the filter in contact with an aqueous solution of areducing agent selected from the group consisting of ascorbic acid,ascorbic acid salts, and mixtures thereof, thereby extracting the iodinefrom the filter and dissolving it in the aqueous solution.
 2. Theprocess according to claim 1, wherein the reducing agent is sodiumascorbate.
 3. The process according to claim 1, wherein the aqueoussolution has a pH of 10 to
 14. 4. The process according to claim 1,wherein the concentration of reducing agent in the aqueous solution isfrom 0.5 to 2 mol.L⁻¹.
 5. The process according to claim 1, wherein thesolid iodine filter comprises a porous mineral solid support based onsilica or alumina impregnated with silver nitrate.
 6. The processaccording to claim 5, wherein the iodine filter comprises silver nitratenot converted into silver iodide and/or silver iodate, the filter issubjected beforehand to a treatment to dissolve the silver nitrate in adilute acidic solution, before carrying out the placing in contact ofthe filter with the aqueous solution of reducing agent.
 7. The processaccording to claim 6, wherein the dissolution of the silver present inthe filter in an aqueous solution is also performed.
 8. The processaccording to claim 7, wherein the dissolution of the silver in anaqueous solution other than that of the reducing agent is performed,after having separated the filter from the aqueous solution of reducingagent, by placing the filter thus separated in contact with asilver-dissolving solution.
 9. The process according to claim 8, whereinthe silver-dissolving solution is a nitric acid solution with a nitricacid concentration of from 2 to 6 mol.L⁻¹.
 10. The process according toclaim 9, wherein a cycle comprising the steps below is performed atleast twice: a) reductive treatment of the iodine filter with an aqueoussolution of the reducing agent, having a pH of from 10 to 14, b)separation of the filter from the aqueous solution, and c) dissolutionof the silver by immersing the filter separated out in step b) in anitric acid solution with a nitric acid concentration of from 2 to 6mol.L⁻¹.
 11. The process according to claim 7, in which the dissolutionof the silver in the aqueous solution of reducing agent issimultaneously performed, and the aqueous solution of reducing agentfurther comprises a silver-complexing agent.
 12. The process accordingto claim 11, wherein the silver-complexing agent is potassium cyanide.13. The process according to claim 1, wherein the aqueous solution hasan OH⁻ ion concentration of up to 2 mol.L⁻¹.
 14. The process accordingto claim 1, wherein the aqueous solution has an OH⁻ ion concentration ofup to 3 mol.L⁻¹.
 15. The process according to claim 9, wherein a cyclecomprising the steps below is performed at least twice: a) reductivetreatment of the iodine filter with an aqueous solution of the reducingagent, having an OH⁻ ion concentration of up to 2 mol.L⁻¹, b) separationof the filter from the aqueous solution, and c) dissolution of thesilver by immersing the filter separated out in step b) in a nitric acidsolution with a nitric acid concentration of from 2 to 6 mol.L⁻.
 16. Theprocess according to claim 9, wherein a cycle comprising the steps belowis performed at least twice: a) reductive treatment of the iodine filterwith an aqueous solution of the reducing agent, having an OH⁻ ionconcentration of up to 3 mol.L⁻¹, b) separation of the filter from theaqueous solution, and c) dissolution of the silver by immersing thefilter separated out in step b) in a nitric acid solution with a nitricacid concentration of from 2 to 6 mol.L⁻¹.